1. Field of Invention
The present invention relates generally to the formation of electrically conductive metallurgical bonds using components having a low melting point constituent and more particularly to a technique for using conductive metal particles having a low melting point constituent in a polymer binder to form a metallurgical bond both between the metal particles themselves and the particles and metal pads on opposing substrates to form a homogenous isotropic electrical connection between the metal pads on the substrates.
2. Background Information
Solder, for many years has been the material of choice in many instances for forming mechanical and electrical bonds between metals such as pads or contacts in electrical circuitry. Solder has been particularly useful in forming bonds between raised pads on two substrates such as joining metal pads on a semi-conductor chip to pads on a chip carrier or circuit board. In such technology, solder is applied to one or both of the pads and caused to melt or reflow and then solidify to form the bond.
Another technique for forming connections between metal pads on opposed substrates, is by the use of conductive adhesives. Such conductive adhesives usually include a high concentration of conductive particles in a thermal setting or thermal plastic binder. Achieving good electrical conductivity in the case of conductive adhesives in the past has depended upon generating sufficient compressive forces between the particles to maintain an intimate surface contact therebetween and at the same time generating sufficient compressive force on the particles against the metal pads on each of the substrates. Such compressive forces are generally obtained through volume shrinkage as the thermal setting binder cures or through solidification of a thermal plastic binder upon cooling after reflow. However, in the case of conductive adhesives, the adhesive is subject to a tendency to creep during thermal cycling and as a result the compressive force may be reduced or lost and hence the mechanical contact between some of the particles may be broken or contact between some of the particles and the pads may be broken, thus resulting in either the total loss of conductivity or substantial increase in the resistance.
A technique for generating metallurgical bonds of a certain type between particles in an adhesive polymer binder or between particles and pads is disclosed in U.S. Pat. No. 5,542,602 issued Aug. 6, 1996 and commonly assigned herewith, and which patent is hereby incorporated herein by reference. In the '602 patent, two different techniques of forming metallurgical bonds each using transient liquid phase (TLP) bonding are shown. In one technique particles having a core material with a coated material on the top thereof and which metals react to form a eutectic are disposed in the adhesive and bonded to each other by means of TLP bonding. This forms a conductive matrix in the polymer after the adhesive has cured or set. In another technique uncoated metal particles are utilized, which form a mono layer between the two pads being joined and an isotropic bond is formed by a TLP reaction between uncoated balls and a coating of metal on the pads to form the eutectic and then a liquid and then a solid phase is formed by the TLP reaction.
In the case of the coated metal balls, a good metallurgical homogeneous metallurgical network is formed with the particles, but a good bond between the homogeneous network and the pads on the opposed substrates is not assured. In the other embodiment wherein the uncoated particles are utilized a good metallurgical bond between the pads is obtained by the use of the uncoated balls but this is limited to a closely spaced isotropic connection with a separation between the pads being required to be no more than a mono layer of the conductive particles.
Therefore, it is desired and a principal objective of this invention to provide a solid metallurgical network of a conductive material connecting pads on adjacent substrates by utilizing TLP technique both for bonding the conductive particles in an adhesive and between the particles and metal on the pads.